U.S. patent application number 11/589857 was filed with the patent office on 2007-07-26 for fixing device and image forming apparatus.
This patent application is currently assigned to KONICA MINOLTA BUSINESS TECHNOLOGIES, INC.. Invention is credited to Yoshihisa Fujimoto, Yutaka Otsuka, Koji Takahashi, Naoki Yamamoto.
Application Number | 20070170172 11/589857 |
Document ID | / |
Family ID | 38284521 |
Filed Date | 2007-07-26 |
United States Patent
Application |
20070170172 |
Kind Code |
A1 |
Otsuka; Yutaka ; et
al. |
July 26, 2007 |
Fixing device and image forming apparatus
Abstract
A Fixing device has a heat roller, a magnetic flux generating
element, and a magnetic flux block section. The magnetic flux block
section for blocking magnetic flux flowing from a magnetic flux
generating element to a heat roller is moved to a position at which
the magnetic flux does not cause the heat roller to generate heat
when rotation of the heat roller stops. The magnetic flux block
section is moved to a position at which the heat roller generates
heat when the heat roller rotates. Therefore, when rotation of the
heat roller stops, even if electric current is applied to the
magnetic flux generating element to generate magnetic flux from the
magnetic flux generating element, heat generation by the magnetic
flux generating element is prevented, so that smoking and firing of
the heating element is suppressed.
Inventors: |
Otsuka; Yutaka;
(Toyokawa-shi, JP) ; Fujimoto; Yoshihisa;
(Toyohashi-shi, JP) ; Takahashi; Koji;
(Toyokawa-shi, JP) ; Yamamoto; Naoki;
(Toyohashi-shi, JP) |
Correspondence
Address: |
BUCHANAN, INGERSOLL & ROONEY PC
POST OFFICE BOX 1404
ALEXANDRIA
VA
22313-1404
US
|
Assignee: |
KONICA MINOLTA BUSINESS
TECHNOLOGIES, INC.
Tokyo
JP
|
Family ID: |
38284521 |
Appl. No.: |
11/589857 |
Filed: |
October 31, 2006 |
Current U.S.
Class: |
219/619 ;
399/328 |
Current CPC
Class: |
G03G 15/2003 20130101;
H05B 6/145 20130101 |
Class at
Publication: |
219/619 ;
399/328 |
International
Class: |
G03G 15/20 20060101
G03G015/20; H05B 6/14 20060101 H05B006/14 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 26, 2006 |
JP |
2006-017229 |
Claims
1. A fixing device, comprising: a magnetic flux generating element
for generating magnetic flux; a rotatable heating element for
generating heat by magnetic flux from the magnetic flux generating
element; a magnetic flux block section placed movably into between
the magnetic flux generating element and the heating element, and
blocking at least part of the magnetic flux flowing from the
magnetic flux generating element to the heating element when being
positioned between the magnetic flux generating element and the
heating element; a movement control section for moving the magnetic
flux block section to a position at which the magnetic flux from
the magnetic flux generating element causes the heating element to
generate no heat when rotation of the heating element stops,
whereas moving the magnetic flux block section to a position at
which the magnetic flux from the magnetic flux generating element
causes the heating element to generate heat when the heating
element rotates.
2. The fixing device according to claim 1, wherein the movement
control section moves the magnetic flux block section in such a way
that an amount of blocked magnetic flux flowing from the magnetic
flux generating element to the heating element decreases as a
rotation speed of the heating element increases.
3. The fixing device according to claim 1, wherein the movement
control section moves the magnetic flux block section in
conjunction with rotation of the heating element.
4. The fixing device according to claim 1, wherein when rotation of
the heating element stops, the movement control section moves the
magnetic flux block section to a position at which the magnetic
flux flowing from the magnetic flux generating element to the
heating element is completely blocked.
5. The fixing device according to claim 1, further comprising
another rotor which rotates following after rotation of the heating
element, wherein the movement control section moves the magnetic
flux block section in conjunction with rotation of the rotor.
6. The fixing device according to claim 1, wherein the magnetic
flux generating element has a coil, and a coil control section is
provided for turning off application of electric current to the
coil when rotation of the heating element stops.
7. The fixing device according to claim 1, wherein the magnetic
flux generating element has a coil, the coil is connected to a
converter, and a converter control section is provided for turning
off application of electric current to the converter when rotation
of the heating element stops.
8. An image forming apparatus comprising the fixing device
according to claim 1.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is based on the application No. 2006-017229
filed in Japan, the entire content of which is hereby incorporated
by reference.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to a fixing device for use in
electrophotographic image forming apparatuses such as copying
machines, laser printers or facsimiles, and relates to an image
forming apparatus with use of the fixing device.
[0003] In recent years, the warm-up period of fixing devices has
become shorter and shorter for the sake of energy saving and
reduction of working hours. That is, a heat roller in the fixing
devices is rapidly heated.
[0004] However, when heating the heat roller in a stopped state,
the temperature rise rate of the heat rollers is increased.
Eventually, it becomes impossible for an overheat protector such as
a thermostat or fuse to prevent the heat rollers from catching
fire.
[0005] Accordingly, as an example of conventional fixing devices,
the heat roller is heated by magnetic flux from an coil while the
heat roller is rotating, wherein magnetic flux is generated by
applying electric current to the coil (JP 2002-82549 A). In other
words, catching fire on the heat roller is suppressed by avoiding
the increase in the temperature rise rate of the heat roller.
[0006] In the conventional fixing device, however, there has been a
problem that the heat roller may be overheated to catch fire when
an electric current is applied to the coil in the stopped state of
the heat roller due to some sort of error.
BRIEF SUMMARY OF THE INVENTION
[0007] An object of the present invention is to provide a fixing
device which prevents the heat roller from catching fire even if
electric current is applied to the coil when the heat roller
stops.
[0008] In order to achieve the above-mentioned object, one aspect
of the present invention provides a fixing device, comprising:
[0009] a magnetic flux generating element for generating magnetic
flux;
[0010] a rotatable heating element for generating heat by magnetic
flux from the magnetic flux generating element;
[0011] a magnetic flux block section placed movably into between
the magnetic flux generating element and the heating element and
blocking at least part of the magnetic flux flowing from the
magnetic flux generating element to the heating element when being
positioned between the magnetic flux generating element and the
heating element;
[0012] a movement control section for moving the magnetic flux
block section to a position at which the magnetic flux from the
magnetic flux generating element causes the heating element to
generate no heat when rotation of the heating element stops,
whereas moving the magnetic flux block section to a position at
which the magnetic flux from the magnetic flux generating element
causes the heating element to generate heat when the heating
element rotates.
[0013] According to the fixing device in the present invention, the
movement control section moves the magnetic flux block section to a
position at which the magnetic flux from the magnetic flux
generating element does not cause the heating element to generate
heat when rotation of the heating element stops. On the other hand,
the movement control section moves the magnetic flux block section
to a position at which the magnetic flux from the magnetic flux
generating element causes the heating element to generate heat when
the heating element rotates. Therefore, when rotation of the
heating element stops, even if electric current is applied to the
magnetic flux generating element to generate magnetic flux from the
magnetic flux generating element, heat generation by the magnetic
flux generating element is prevented, so that smoking and firing of
the heating element can be suppressed. This makes it possible to
achieve a high-security fixing device.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] The present invention will become more fully understood from
the detailed description given hereinbelow and the accompanying
drawings which are given by way of illustration only, and thus are
not limitative of the present invention, and wherein:
[0015] FIG. 1 shows a cross sectional view of a fixing device in a
first embodiment of the present invention;
[0016] FIG. 2 is a side view of the fixing device as viewed from an
axial end side of a heat roller;
[0017] FIG. 3 is a partially cut perspective view of a damper;
[0018] FIG. 4A is an operation explanation view of a magnetic flux
block section to show a position thereof in the case where the
rotation speed of the heat roller is low;
[0019] FIG. 4B is an operation explanation view of the magnetic
flux block section to show a position thereof in the case where the
rotation speed of the heat roller is high;
[0020] FIG. 5 is a simplified structure view of a fixing device in
a second embodiment of the present invention;
[0021] FIG. 6 is a simplified structure view of a fixing device in
a third embodiment of the present invention; and
[0022] FIG. 7 is a simplified structure view of an image forming
apparatus according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0023] Hereinbelow, the invention will be described in detail in
conjunction with the embodiments with reference to the
drawings.
First Embodiment
[0024] FIG. 1 is a cross sectional structure view of a fixing
device in a first embodiment of the present invention. The fixing
device has a heat roller 1, a pressure roller 2 and a magnetic flux
generating element 3. The heat roller 1 serves as a rotatable
heating element. The pressure roller 2 serves as a pressing element
coming into pressure contact with the heat roller 1. The magnetic
flux generating element 3 is placed outside the heat roller 1 for
generating magnetic flux to make the heat roller 1 generate
heat.
[0025] A magnetic flux block section 5 is placed between the
magnetic flux generating element 3 and the heat roller 1 in such a
way that the magnetic flux block section 5 can move between the
magnetic flux generating element 3 and the heat roller 1 in the
rotation direction (circumferential direction) of the heat roller
1. A movement control section 4 is provided to move the magnetic
flux block section 5 to a specified position in the rotation
direction (circumferential direction) of the heat roller 1.
[0026] The heat roller 1 and the pressure roller 2 contact with
each other to fix a toner t of a recording member S while
transporting the recording member S. More specifically, the heat
roller 1 and the pressure roller 2 contact with each other to form
a nip section N, wherein the heat roller 1 generates heat by
magnetic flux from the magnetic flux generating element 3, and then
the nip section N transports a recording member S while melting and
fixing the toner t on the recording member S.
[0027] The recording member S is exemplified by sheets such as
paper sheets and OHP sheets. The toner t is made of thermally
meltable materials such as resins, magnetic materials and
colorants.
[0028] The heat roller 1 and the pressure roller 2 are placed
parallely facing each other. Both the ends of the respective
rollers are rotatably supported by unshown bearing members. The
heat roller 1 and the pressure roller 2 freely come close to or
away from each other.
[0029] The pressure roller 2 is rotationally driven
counterclockwise as shown by an arrow at a specified peripheral
velocity by a motor 9 serving as a drive section. The heat roller 1
rotates in a clockwise direction as shown by an arrow, following
after the rotation of the pressure roller 2, wherein the rotation
results from a friction force attained by pressure contact between
the heat roller 1 and the pressure roller 2 at the nip section
N.
[0030] The heat roller 1 has an outer diameter of 40 mm for
example. The heat roller 1 has a support layer 11, a heat
insulating layer 12, an electromagnetic induction heat generating
layer 13, an elastic layer 14 and a release layer 15 which are
placed in sequence from the inside toward the outside in the radial
direction. Although the heat roller 1 has the elastic layer 14 for
supporting color images in this embodiment, the heat roller 1 has
only to have at least the support layer 11, the heat insulating
layer 12, the electromagnetic induction heat generating layer 13
and the release layer 15.
[0031] For the support layer 11, a cylinder shaft may be used,
which is made of steel and has an outer diameter of 30 mm for
example. A heat-resistant molded pipe made of, for example,
polyphenylene sulfide (PPS) may also be used for the support layer
11 as long as the material strength thereof is ensured. However,
nonmagnetic material such as aluminum is preferably used for the
support layer 11 in order to prevent the shaft from generating heat
since the nonmagnetic material hardly generates heat by
electromagnetic induction.
[0032] The heat insulating layer 12 is for insulating and retaining
heat generated by the electromagnetic induction heat generating
layer 13. The heat insulating layer 12 is made of sponges (heat
insulating structures) made from rubber materials and resin
materials having not only heat resistance but also elasticity. For
the heat insulating layer 12, the use of sponges (heat insulating
structures) made from rubber materials and resin materials having
high heat resistance and high elasticity makes it possible to
reliably insulate the electromagnetic induction heat generating
layer 13 and to allow deflection of the electromagnetic induction
heat generating layer 13. Thus, it becomes possible to enhance
performance of sheet discharge and performance of recording member
separation by increasing a width size of the nip section N and by
making the roller hardness of the heat roller 1 lower than that of
the pressure roller 2. In the case where the heat insulating layer
12 is made of a silicon sponge material for example, its thickness
is set at 2 mm to 10 mm, preferably 3 mm to 7 mmm and its hardness
is set at 20 to 60 degrees, preferably 30 to 50 degrees according
to an Asker rubber hardness meter.
[0033] The electromagnetic induction heat generating layer 13 is,
for example, an endless electroformed nickel belt layer with a
thickness of 10 to 100 .mu.m and preferably 20 to 50 .mu.m. As
other materials of the electromagnetic induction heat generating
layer 13, it is acceptable to use magnetic materials (magnetic
metals), such as magnetic stainless steels, which have a relatively
high magnetic permeability .mu. and an appropriate resistivity
.rho.. Even if materials are nonmagnetic, materials having electric
conductivity such as nonmagnetic metals may be used by forming them
into thin films for example. Moreover, materials in which heat
generating particles are dispersed in resin may be used for the
electromagnetic induction heat generating layer 13. The
electromagnetic induction heat generating layer 13 made of
resin-based materials makes it possible to further enhance the
separating performance of the recording member S. The
electromagnetic induction heat generating layer 13 is bonded onto
the heat insulating layer 12.
[0034] In the electromagnetic induction heat generating layer 13,
an eddy current is generated by magnetic flux from the magnetic
flux generating element 3 so as to generate Joule heat, by which
the heat roller 1 is heated. This heating is referred to as
electromagnetic induction heating.
[0035] The elastic layer 14 is made of a rubber material or a resin
material having not only heat resistance but also elasticity so as
to promotes adhesion between the recording member S and the surface
of the heat roller 1. Heat-resistant elastomer such as silicon
rubber or fluorocarbon rubber is used as the elastic layer 14,
which rubber can withstand use at fixing temperatures. Various
fillers may be mixed with the elastic layer 14 for the purpose of
enhancing thermal conductivity, reinforcement or the like. Examples
of thermally conductive particles include diamond, silver, copper,
aluminum, marble and glass. Practical examples thereof include
silica, alumina, magnesium oxide, boron nitride and beryllium
oxide.
[0036] The thickness of the elastic layer 14 should preferably be,
for example, 10 .mu.m to 800 .mu.m and more preferably be 100 .mu.m
to 300 .mu.m. If the thickness of the elastic layer 14 is less than
10 .mu.m, it is difficult to attain elasticity in the thickness
direction. On the other hand, if the thickness of the elastic layer
14 exceeds 800 .mu.m, it is difficult for heat generated in the
electromagnetic induction heat generating layer 13 to reach the
outer peripheral face of the heat roller 1. As the result, there is
a tendency for the thermal efficiency to deteriorate.
[0037] The hardness of the elastic layer 14 should be 1 to 80
degrees and preferably 5 to 30 degrees in JIS hardness scale. Then,
it is possible to prevent failure in fixing of toner t while
preventing degradation in strength and failure in adhesion of the
elastic layer 14. In this case, the elastic layer 14 is preferably
made of silicon rubber. Specifically, the silicon rubber are
one-component, two-component or three or more-component silicon
rubbers, LTV-type, RTV-type or HTV-type silicon rubbers, and
condensation-type or addition-type silicon rubbers. In this
embodiment, the elastic layer 14 is a silicon rubber having a JIS
hardness of 10 degree and a thickness of 200 .mu.m.
[0038] The release layer 15 is for enhancing the releasing property
of the surface of the fixing roller 1. The release layer 15
withstands any uses at fixing temperatures and has the property of
releasing toner. For example, silicon rubber, fluorocarbon rubber
or fluorocarbon resin such as PFA, PTFE, FEP or PFEP is used for
the release layer 15. The thickness of the release layer 15 is
preferably 5 .mu.m to 100 .mu.m and more preferably 10 .mu.m to 50
.mu.m. In order to enhance an interlayer adhesion force, adhesion
processing may be performed by using a primer or the like.
According to need, the release layer 15 may contain conductive
material, abrasion-resistant material and/or good thermal
conductive material as fillers.
[0039] The pressure roller 2 has for example 30mm of outer
diameter, and has a support layer 21, a heat insulating layer 22
and a release layer 23, which are placed in sequence from the
inside toward the outside of the pressure roller 2 in the radial
direction. The support layer 21 is made out of, for example, a
steel hollow shaft having an outer diameter of 20 mm. The heat
insulating layer 22 is made out of, for example, a silicon sponge
rubber formed by foaming silicon rubber having a thickness of 3 to
10 mm. The release layer 23 is made out of, for example,
fluorocarbon resin such as PTFE or PFA having a thickness of 10 to
50 .mu.m.
[0040] The material of the support layer 21 may be a heat-resistant
molded pipe made of, for example, PPS (polyphenylene sulfide) as
long as the strength can be ensured. However, in order to prevent
the shaft from generating heat, it is preferable to use nonmagnetic
material such as aluminum which is less heated by electromagnetic
induction.
[0041] The pressure roller 2 is pressed toward the heat roller 1
with a load of 300 to 530N. In this case, a width size of the nip
section N (i.e. size thereof in the transportation direction of the
recording member S) is about 5 to 15 mm. A longitudinal size of the
nip section N (i.e. size thereof in axial direction of the heat
roller 1) is about 340 mm. The width size of the nip section N may
be changed by changing the load.
[0042] The magnetic flux generating element 3 faces the heat roller
1 in such a way that the magnetic flux generating element 3 extends
along the longitudinal direction of the heat roller 1. The magnetic
flux generating element 3 has a magnetic core 32 and a coil 31. The
magnetic core 32 is placed outside along the outer face of the heat
roller 1. The coil 31 is placed on a surface of the core 32 which
surface faces the heat roller 1.
[0043] The core 32 is shaped into a circular-arc extending along
the outer face of the heat roller 1 in a transverse sectional view.
The coil 31 has such a structure that a conductive wire is wound
along the inner face of the coil 31 so as to be longer in the axial
direction of the heat roller 1. The coil 31 has a roughly
trapezoidal shape in the transverse sectional view.
[0044] The magnetic core 32 having high magnetic permeability and
low loss are generally used. The magnetic core 32 is used so as to
increase the efficiency of magnetic circuits and to shield
magnetism. Ferrite cores are usually used for material of the
magnetic core 32. However, in the case where alloy such as
permalloy is used, the magnetic core 32 may have a laminated
structure since radio frequencies may cause an eddy current loss to
be increased in the magnetic core 32. In addition, in the case of
using resin material with magnetic powders dispersed therethrough,
the magnetic permeability becomes relatively low. Thereby, however,
the magnetic core 32 is freely shaped.
[0045] The central portion of the magnetic core 32 in the rotation
direction of the heat roller 1 has a protrusion protruding toward
the heat roller 1, so that the heat generation efficiency of the
heat roller 1 is enhanced. The protrusion may be removed if there
is a mechanism to provide sufficient magnetic shielding in the
magnetic circuit section which is formed by the coil 31 and the
core 32.
[0046] An RF converter 7 is connected to the coil 31 to supply a
radio frequency power of 100 to 2000 W, for example. A Litz wire,
which is formed by bunching or twisting several dozen to several
hundred thin wires, is used as the coil 31. The Litz wire is coated
with heat-resistant resin in consideration of heat conducted to the
winding wires.
[0047] An alternating current of, for example, 10 to 100 kHz is
applied to the coil 31 by the converter 7. The magnetic flux
induced by the alternating current penetrates the electromagnetic
induction heat generating layer 13 of the heat roller 1 from the
core 32. This causes eddy current to flow through the
electromagnetic induction heat generating layer 13. Thereby, the
electromagnetic induction heat generating layer 13 generates Joule
heat. The heat roller 1 is heated by the heat generated in the
electromagnetic induction heat generating layer 13.
[0048] A temperature sensor 6 for detecting the temperature of the
heat roller 1 is placed outside the heat roller 1. The temperature
sensor 6 is, for example, a noncontact-type thermister and placed
away from the surface of the heat roller 1.
[0049] The temperature sensor 6 is connected to a main control
section 8. The main control section 8 controls the converter 7
based on a surface temperature detection signal of the heat roller
1 from the temperature sensor 6. Specifically, the surface
temperature of the heat roller 1 is automatically controlled to
become a specified constant temperature by increasing or decreasing
power supply to the coil 31 in the converter 7. The main control
section 8 has a converter control section 8a which turns off
application of electric current to the converter 7 when rotation of
the heat roller 1 stops.
[0050] The magnetic flux block section 5 blocks at least part of
magnetic flux flowing from the magnetic flux generating element 3
to the heat roller 1 when the magnetic flux block section 5
positions between the magnetic flux generating element 3 and the
heat roller 1. The magnetic flux block section 5 is made of
nonmagnetic metal materials having good conductivity, such as
alloys of aluminum, copper, silver or the like. Thereby, the
magnetic flux is blocked to suppress heating caused by induction
current. The magnetic flux block section 5 has a plate shape curved
along the outer face of the heat roller 1.
[0051] In the rotation direction of the heat roller 1, the surface
size of the magnetic flux block section 5 is almost identical to
the inner surface size of the magnetic flux generating element 3.
In the axial direction of the heat roller 1, the magnetic flux
block section 5 can exist outside the end section of the heat
roller 1.
[0052] FIG. 2 shows a side view of the end section of the heat
roller 1. As shown in FIG. 2, the movement control section 4 moves
the magnetic flux block section 5 to the position where the
magnetic flux from the magnetic flux generating element 3 does not
cause the heat roller 1 to generate heat when the heat roller 1
does not rotate. On the other hand, the movement control section 4
moves the magnetic flux block section 5 to the position where the
magnetic flux from the magnetic flux generating element 3 causes
the heat roller 1 to generate heat when the heat roller 1
rotates.
[0053] More particularly, when the heat roller 1 stops, the
movement control section 4 moves the magnetic flux block section 5
to the position where the magnetic flux flowing from the magnetic
flux generating element 3 to the heat roller 1 is completed
blocked. The state of the magnetic flux block section 5 at this
point of time is referred to as a close state of the magnetic flux
block section 5, in which the magnetic flux block section 5 covers
the entire region between the heat roller 1 and the magnetic flux
generating element 3.
[0054] When the heat roller 1 rotates, the movement control section
4 moves the magnetic flux block section 5 in an arrow direction in
conjunction with the rotation of the heat roller 1 in the arrow
direction. The state of the magnetic flux block section 5 at this
point of time is referred to as an open state of the magnetic flux
block section 5.
[0055] More specifically, the movement control section 4 has a
damper 40 mounted on a shaft section la of the heat roller 1, a
coupling rod 41 fixed onto the damper 40, the magnetic flux block
section 5 fixed onto the coupling rod 41 and a tension spring 42
attached to the magnetic flux block section 5.
[0056] A turning effect of the shaft section la of the heat roller
1 is transmitted to the magnetic flux block section 5 through the
damper 40. The magnetic flux block section 5 moves along a rail 43
fixed onto an unshown casing. The rail 43 has a plate shape curved
along the outer face of the heat roller 1. The rail 43 is placed
outside the end of the heat roller 1 so as not to disturb rotation
of the heat roller 1. The rail 43 is not an essential
component.
[0057] One end of the tension spring 42 is fixed onto an unshown
casing, while the other end of the tension spring 42 is fixed onto
the magnetic flux block section 5. The tension spring 42 constantly
biases the magnetic flux block section 5 in such a way that the
magnetic flux block section 5 moves to the position at which the
heat roller 1 does not generate heat. Basically, the tension spring
42 pulls the magnetic flux block section 5 in a direction opposite
to the direction that the magnetic flux block section 5 moves in
conjunction with the rotation of the heat roller 1. More
particularly, the tension spring 42 puts the magnetic flux block
section 5 in the close state when the heat roller 1 stops.
[0058] As shown in FIG. 3, the damper 40 is a disc damper. The
damper 40 has a housing 45 and a rotor 47 mounted through a sheet
46. The housing 45 has a frame 48 onto which the coupling rod 41 is
fixed. The shaft section la of the heat roller 1 is fixed onto the
rotor 47. The sheet 46 is fixed onto the housing 45.
[0059] Each of O-rings 49 is partially provided between the sheet
46 and the rotor 47, between the frame 48 and the rotor 47 and
between the frame 48 and the sheet 46.
[0060] Oil is filled between the rotor 47 and the sheet 46. A
torque is generated by viscous resistance of the oil generated
during relative rotation between the rotor 47 and the sheet 46.
[0061] More particularly, the sheet 46, the housing 45 and the
coupling rod 41 rotate in the same direction as the rotor 47 by the
viscosity of the oil when the rotor 47 rotates together with the
shaft section la of the heat roller 1.
[0062] A higher rotation speed of the heat roller 1 increase the
torque of the damper 40. On the other hand, a lower rotation speed
of the heat roller 1 decreases the torque of the damper 40.
[0063] More particularly, as shown in FIGS. 4A and 4B, the movement
control section 4 moves the magnetic flux block section 5 in such a
way that an amount of a blocked magnetic flux flowing from the
magnetic flux generating element 3 to the heat roller 1 decreases
as the rotation speed of the heat roller 1 increases.
[0064] FIG. 4A shows the location of the magnetic flux block
section 5 in the case of a peripheral velocity on the outer face of
the heat roller 1 being 30 mm/s. The magnetic flux block section 5
is located in a half-close state and covers a half of the region
between the heat roller 1 and the magnetic flux generating element
3.
[0065] FIG. 4B shows the location of the magnetic flux block
section 5 in the case of a peripheral velocity on the outer face of
the heat roller 1 being 60 mm/s. The magnetic flux block section 5
is located in a fully open state and exposes the entire region
between the heat roller 1 and the magnetic flux generating element
3.
[0066] Therefore, when the peripheral velocity of the outer face of
the heat roller 1 becomes double, the amount of the magnetic flux
flowing from the magnetic flux generating element 3 to the heat
roller 1 becomes double.
[0067] When the heat roller 1 rotates, force larger than the force
generated by the tension spring 42 is generated in the damper 40.
When rotation of the heat roller 1 stops, the tension spring 42
pulls the magnetic flux block section 5 against the viscous
resistance in the damper 40.
[0068] Description is now given of operations of the
above-structured fixing device.
[0069] An operation for heating the surface of the heat roller 1
and the surface of the pressure roller 2 up to a printable
temperature after the fixing device is powered on is called a
warm-up operation. A time taken for the operation is called a
warm-up time. The warm-up operation also runs at the times of power
cycle, recovery from jamming processing, cover close, and recovery
from a sleep mode.
[0070] In the warm-up operation, for gaining the printable
temperature, electric current is applied to the coil 31 by the main
control section 8 shown in FIG. 1, and thereby the electromagnetic
induction heat generating layer 13 of the heat roller 1 generates
heat.
[0071] Then, the pressure roller 2 is rotated by the motor 9 so as
to rotate the heat roller 1 following after rotation of the
pressure roller 2, and thereby heat of the heat roller 1 is
transmitted to the surface of the pressure roller 2. This rotation
allows the surface of the heat roller 1 and the surface of the
pressure roller 2 to be heated to the printable temperature in
shorter period of time.
[0072] The temperature of the heat roller 1 is detected in a
noncontact way by the temperature sensor 6. The temperature of the
heat roller 1 is corrected by a temperature shift caused by the
noncontact. When the corrected temperature reaches a specified
printable temperature, the fixing device becomes "ready" which
indicate a printable state. Specifically, the fixing device becomes
"ready" when the corrected temperature, which is based on the
detected temperature of the temperature sensor 6, reaches for
example 190.degree. C.
[0073] When a print signal is not present, the system is put in a
print standby state. When the print signal is present, a printing
operation starts. During the standby state, current application is
controlled by the main control section 8 so as to keep a specified
set temperature in the state of rotation or no rotation of the heat
roller 1. The specified set temperature of the heat roller 1 is,
for example, 190.degree. C.
[0074] At the time of printing, the temperature of the pressure
roller 2 have been increased by conducting heat of the heat roller
1 to the pressure roller 2, before the recording member S enters
the fixing device. In this case, the temperature rise speed is
about 50.degree. C./s when the heat roller 1 is in a stopped state,
whereas the temperature rise speed is about 25.degree. C./s when
the heat roller 1 rotates.
[0075] The fixing device is equipped with an unshown excessive
temperature rise prevention device as a safety device. The
excessive temperature rise prevention device is a thermostat for
example. The thermostat is placed in the vicinity of the magnetic
flux generating element 3 so as to detect the surface temperature
of the heat roller 1 in a noncontact state. In consideration of
preventing malfunction other than excessive temperature rise, the
temperature of the thermostat is set at, for example, 210.degree.
C.
[0076] The operation of the thermostat is delayed in response to
rapid temperature rise of the heat roller 1. This is because the
thermostat is placed in the noncontact state relative to the heat
roller 1 and because the large thermal capacity or endothermic
properties of the thermostat, and-so on.
[0077] For example, in the case of the temperature rise speed of
the heat roller 1 being 20.degree. C./s, the thermostat operates
when the temperature of the heat roller 1 reaches 420.degree. C. In
the case of the temperature rise speed of the heat roller 1 being
25.degree. C./s, the thermostat operates when the temperature of
the heat roller 1 reaches 450.degree. C.
[0078] In the case of the temperature rise speed of the heat roller
1 being 30.degree. C./s, the thermostat operates when the
temperature of the heat roller 1 reaches 500.degree. C. In the case
of the temperature rise speed of the heat roller 1 being 35.degree.
C./s, the thermostat operates when the temperature of the heat
roller 1 reaches 570.degree. C.
[0079] The thermostat cannot prevent the heat roller 1 from
catching fire in the case of the temperature rise speed of the heat
roller 1 in the stopped state being about 50.degree. C./s, and in
the case where an ignition temperature of the heat roller 1 is, for
example, 500.degree. C.
[0080] However, according to the fixing device having the
above-stated structure, when rotation of the heat roller 1 stops,
the movement control section 4 moves the magnetic flux block
section 5 to a position at which magnetic flux from the magnetic
flux generating element 3 causes the heat roller 1 to generate no
heat. On the other hand, when the heat roller 1 rotates, the
movement control section 4 moves the magnetic flux block section 5
to a position at which the magnetic flux from the magnetic flux
generating element 3 causes the heat roller 1 to generate heat
Thereby, the magnetic flux generating element 3 is prevented from
heat generation even if electric current is applied to the magnetic
flux generating element 3 to generate magnetic flux from the
magnetic flux generating element 3 when rotation of the heat roller
1 stops. Thus, smoking and firing of the heat roller 1 can be
suppressed, so that it becomes possible to achieve a high-security
fixing device.
[0081] Heat generation of the magnetic flux generating element 3 is
reliably prevented when rotation of the heat roller 1 stops. This
is because when rotation of the heat roller 1 stops, the movement
control section 4 moves the magnetic flux block section 5 to a
position at which magnetic flux flowing from the magnetic flux
generating element 3 to the heat roller 1 is completely
blocked.
[0082] The magnetic flux block section 5 has good responsiveness to
the rotation of the heat roller 1 because the movement control
section 4 moves the magnetic flux block section 5 in conjunction
with the rotation of the heat roller 1. Thus, it becomes possible
to swiftly and reliably move the magnetic flux block section 5 to
the position at which the heat roller 1 generates no heat when
rotation of the heat roller 1 stops.
[0083] The movement control section 4 constantly biases the
magnetic flux block section 5 toward the position at which the heat
roller 1 does not generate heat. Thereby, the magnetic flux block
section 5 is prevented from staying at the position which causes
the heat roller 1 to generate heat, so that the heat roller 1 is
more reliably prevented from smoking and firing.
[0084] The movement control section 4 moves the magnetic flux block
section 5 in such a way as to decrease an amount of a blocked
magnetic flux flowing from the magnetic flux generating element 3
to the heat roller 1, as a rotation speed of the heat roller 1
increases. Therefore, a higher rotation speed of the heat roller 1
increases an amount of the magnetic flux flowing to the heat roller
1, so that a heating value of the heat roller 1 can be increased.
Therefore, regardless of the rotation speed of the heat roller 1,
it becomes possible to keep the temperature rise speed of the heat
roller 1 constant. Generally, in the case where the amount of
magnetic flux flowing to the heat roller 1 is constant, the higher
rotation speed of the heat roller 1 reduces the temperature rise
speed of the heat roller 1.
[0085] Moreover, the converter control section 8a is provided in
order to turn off application of electric current to the converter
7 when rotation of the heat roller 1 stops. This makes it possible
to reliably prevent generation of magnetic flux from the magnetic
flux generating element 3 when rotation of the heat roller 1
stops.
Second Embodiment
[0086] FIG. 5 shows a fixing device in a second embodiment of the
present invention. The second embodiment is different, compared to
the first embodiment, in that the movement control section 4 moves
the magnetic flux block section 5 in conjunction with the rotation
of the pressure roller 2.
[0087] In other words, the damper 40 is mounted on a shaft section
2a of the pressure roller 2. A turning force of the shaft section
2a of the pressure roller 2 is transmitted to the magnetic flux
block section 5 through the damper 40. The magnetic flux block
section 5 moves in conjunction with the pressure roller 2 which
rotates following after the rotation of the heat roller 1.
[0088] Therefore, it is possible to determine whether or not the
heat roller 1 rotates, based on whether or not the pressure roller
2 rotates. In addition, the pressure roller 2 has a function to
transport the recording member S in pressure contact with the heat
roller 1. Therefore, the pressure roller 2 may both fulfill the
above-stated function and determine whether or not the heat roller
1 rotates, so that it becomes possible to reduce the number of
component parts.
[0089] It is acceptable to mount the movement control section 4 on
other rotor, which rotates following after the heat roller 1, than
the pressure roller 2 in order to move the magnetic flux block
section 5 in conjunction with the other rotor. The other rotor
includes, for example, a rotor having a function of a cleaning
rotor which cleans the heat roller 1 and the pressure roller 2 in
the contact state with the heat roller 1 and the pressure roller
2.
Third Embodiment
[0090] FIG. 6 shows a fixing device in a third embodiment of the
present invention. In the third embodiment, as compared with the
first embodiment, the main control section 8 has a coil control
section 8b instead of the converter control section 8a. The coil
control section 8b turns off application of electric current to the
coil 31 when rotation of the heat roller 1 stops.
[0091] This makes it possible to reliably prevent generation of
magnetic flux from the magnetic flux generating element 3 when
rotation of the heat roller 1 stops. Also, the main control section
8 may have the coil control section 8b together with the converter
control section 8a.
Fourth Embodiment
[0092] FIG. 7 is a simplified structure view of an image forming
apparatus in another embodiment of the present invention. The image
forming apparatus includes an imaging device 80 for forming an
image by attaching unfixed toner t on the recording member S, and
includes a fixing device 81 of the first embodiment for melting the
toner t and fixing it onto the recording member S. The image
forming apparatus is an electrophotographic four-color printer.
[0093] The image forming device 80 includes an intermediate
transfer belt 61, four image forming units 51, a primary transfer
section 62, and a secondary transfer section 63. The four image
forming units 51 are disposed along the intermediate transfer belt
61 for forming toner images. The primary transfer section 62
transfers the toner images formed by the respective image forming
units 51 onto the intermediate transfer belt 61. The secondary
transfer section 63 transfers the images transferred onto the
intermediate transfer belt 61 onto the recording member S.
[0094] The image forming unit 51 forming a black (BK) toner image,
the image forming unit 51 for forming a yellow (Y) toner image, the
image forming unit 51 for forming a magenta (M) toner image and the
image forming unit 51 for forming a cyan (C) toner image are
disposed in sequence along the upper stream toward the downs stream
of the intermediate transfer belt 61.
[0095] Each of the image forming unit 51 includes a photoreceptor
drum 52, a charging section 53 for uniformly charging the
photoreceptor drum 52, an exposure section 59 for applying image
exposure to the charged photoreceptor drum 52, and a development
section 54 for developing an electrostatic latent image formed
through exposure with the toner of respective colors.
[0096] The image forming apparatus includes a control device 68 for
controlling the entire image forming apparatus and an exposure
control device 69 for receiving signals corresponding to images
sent from the control device 68. The exposure control device 69
drives each of the exposure sections 59 corresponding to the
respective colors.
[0097] Description is now given of the actions of the image forming
apparatus.
[0098] A toner image developed on the photoreceptor drum 52 of an
image forming unit 51 is primary-transferred onto the intermediate
transfer belt 61 at a position of contact with the intermediate
transfer belt 61 by the primary transfer section 62.
[0099] The toner images transferred onto the intermediate transfer
belt 61 are superimposed on top thereof with respective colors as
the toner image passes the respective image forming units 51.
Finally, a full-color toner image is formed on the intermediate
transfer belt 61.
[0100] Thereafter, the full-color toner image on the intermediate
transfer belt 61 is collectively subjected to secondary transfer
onto the recording member S on the down stream side of the
intermediate transfer belt 61 by the secondary transfer section
63.
[0101] Then, the recording member S passes through the fixing
device 81 placed on the downstream side of a transportation path of
the recording member S, and thereby the toner image is fixed.
Thereafter, the recording member S is discharged onto a discharge
tray 66.
[0102] The recording member S is housed in a cassette 67 in a
lowermost section of the image forming apparatus and is transported
one by one from the cassette 67 to the secondary transfer section
63.
[0103] After the primary transfer, the toner remaining on the
photoreceptor drum 52 is removed by a cleaning section 55, whish is
placed on the downstream side, so as to be collected from the lower
side of the cleaning section 55.
[0104] After the secondary transfer, the toner remaining on the
intermediate transfer belt 61 is removed from the intermediate
transfer belt 61 by a cleaning blade 65, and is transported by an
unshown transportation screw so as to be collected in an unshown
waste toner container.
[0105] The thus-structured image forming apparatus is provided with
the fixing device 81 of the first embodiment of the present
invention, so that it becomes possible to prevent smoking and
firing from the fixing device 81 to ensure high security. The
fixing device in either the second embodiment or the fifth
embodiment may be employed as a fixing device of the image forming
apparatus.
[0106] It should be noted that the present invention is not limited
to the above-stated embodiments. For example, the damper 40 may be
a pressure damper instead of the disc damper. The pressure damper
generates a torque by using the pressure of oil inside thereof. The
pressure damper has a mechanism to adjust a flow amount and a flow
direction of the oil so as to change the torque according to the
rotation direction and the rotation speed of the roller.
[0107] Also, the heating element and the pressing element may be a
belt instead of the roller. Further, the magnetic flux generating
element 3 may be placed inside the heating element.
[0108] Furthermore, when rotation of the heat roller 1 stops,
magnetic flux need only be blocked by the magnetic flux block
section 5 to the extent that the heat roller 1 does not generate
heat. Moreover, the magnetic flux block section 5 may electrically
be moved by the movement control section 4.
[0109] The image forming apparatus may be any one of
monochrome/color copiers, printers, facsimiles and compound
machines thereof.
[0110] The invention being thus described, it will be obvious that
the invention may be varied in many ways. Such variations are not
be regarded as a departure from the spirit and scope of the
invention, and all such modifications as would be obvious to one
skilled in the art are intended to be included within the scope of
the following claims.
* * * * *